Pentafluoroethane is one of the possible substitutes for chlorofluorocarbons (CFCs) to which the Montreal protocol applies and which are characterized by an exceptionally long life span enabling them to reach the upper layers of the atmosphere and thus to contribute under the influence of UV radiation to the destruction of the ozone layer. It is therefore obvious that their substitutes must contain only traces of these CFCs, depending on the various processes for obtaining them.
The substitutes are generally obtained either by suitable fluorination methods which are not highly selective and can generate perhalogenated compounds of the CFC type by disproportionation, i.e. from CFCs themselves by reduction methods, in practice by hydrogenolysis reactions. It is in this way that pentafluoroethane (R125) can be prepared by fluorination of perchloroethylene or intermediate fluorination products thereof such as dichlorotrifluoroethane (R123) and chlorotetrafluoroethane (R124), or by hydrogenolysis of chloropentafluoroethane (R115). In both cases, the R125 produced contains significant quantities of R115 that it is advisable to remove as completely as possible, R115 being a CFC.
However, the existence of an R115/R125 azeotrope containing 21% by weight R115 (see U.S. Pat. No. 3,505,233) with a boiling point (−48.5° C. under 1.013 bar) very close to that of R125 (−481° C.) makes the complete separation of R115 and R125 by distillation practically impossible unless technically complex processes are used such as azeotropic distillation at different pressures as described in U.S. Pat. No. 5,346,595. The removal of R115 from R125 can therefore only be done by a chemical route or by physical methods involving an entrainer.
In patent application EP 0 508 631 which describes the production of hydrofluorocarbons (HFCs) by chemical reduction in liquid phase of chlorinated, brominated or iodinated compounds with a metal hydride or a complex of such a hydride, it is indicated that this process can be useful for purifying certain HFCs such as pentafluoroethane. To the same end, the Japanese patent application (Kokai) published under No, 2001414/90 uses metallic redox pairs in a solvent medium. Other techniques such as that described in the Journal of Fluorine Chemistry, 1991 vol. 55, p. 105-107, use organic reducing agents such as ammonium formate in a DMF medium and in the presence of ammonium persulphate.
These processes which require reagents that are difficult to handle (metal hydrides) or likely to pose effluent problems, are not very compatible with the industrial production of R125 in significant tonnages.
For the industrial production of R125, the extractive distillation technique appears to be the ideal process for removing the residual R115.
In an extractive distillation process, separation of the constituents of a binary mixture is carried out using a column called an extraction column comprising successively, from the boiler to the top, three sections, one for stripping, the second for absorption and the third for recovery.
The binary mixture to be fractionated is injected at the top of the stripping section while the entrainer acting as a selective solvent or extractant is introduced at the top of the absorption section so as to travel in the liquid state from its point of introduction to the boiler.
The third section called recovery section serves to separate by distillation the least absorbed constituent, from traces of solvent entrained under the effect of the non zero vapour pressure thereof.
Document EP 0 669 302 describes a process for the purification of pentafluoroethane containing chloropentafluoroethane by extractive distillation, the extractant being a C5 to C8 alkane or a cycloalkane.
Document FR 2 758 137 describes a process for the purification of pentafluoroethane containing chloropentafluoroethane by extractive distillation, the extractant being a C5 to C8 perfluoroalkyl halide.
Document FR 2 730 228 describes a process for the purification of pentafluoroethane containing chloropentafluoroethane by extractive distillation, the extractant being perchloroethylene.
Document WO 96/06063 describes a process for the purification of pentafluoroethane comprising bringing pentafluoroethane into contact with a liquid polar organic compound comprising at least one nitrogen atom and/or an oxygen atom.
Certain of these extractants do not satisfy the conditions imposed in terms of environmental requirements.
Extractants have been found making it possible to satisfy environmental requirements and having an improved selectivity and/or capacity.